1. Field of the Invention
This invention relates to presses of the type which use a toggle mechanism to obtain a clamping force and which use a hydraulic system to actuate the toggle mechanism, and particularly to means for indicating the loading pressure of such presses. While this invention may relate to various types of toggle presses used in a wide variety of applications, the invention has special utility in apparatus for the manufacture of plastic articles, such as injection molding machines, and it will be described in the context of such machines.
2. Description of the Prior Art
In plastic injection molding machines, various types of apparatus are used for clamping the mold together during the injection molding process. Among these types of apparatus are hydraulically actuated toggle mechanisms. Such toggle mechanisms provide a mechanical advantage allowing the use of a hydraulic system having a smaller capacity pump than the hydraulic systems used in a press which is closed directly by a hydraulic cylinder. Toggle mechanisms also provide positive lock capabilities. The injection force is absorbed by the linkage system of the press rather than directly by the hydraulic cylinder, and no hydraulic pressure is required in the clamp cylinder, so that the same hydraulic pumps can be used for both the injection cylinders and the clamp cylinders.
In the injection molding process, plastic material, usually in pellet form, is heated in an injection chamber until it reaches a viscous state in which it can be forced to flow into the mold cavities. The press which is used to close the mold halves and clamp the mold together must provide sufficient force or tonnage to prevent the fluid plastic moving at high pressures from forcing the mold halves apart. If the mating surfaces of the mold are forced apart even a few thousandths of an inch, fluid plastic will flow or flash across the mating area. The amount of clamping force required depends upon the projected area of the molded part. Concern should also be given to the viscosity of the plastic at the final mold fill stage.
The force or tonnage exerted by the press on the mold is extremely important in the injection molding process. The tonnage must exceed the pressure exerted by the plastic to resist separation of the mold and flashing of the material. However, the amount of tonnage exerted by the mold should not be excessively greater than that required to resist separation of the mold halves in order to avoid damage to the mold. In many instances, such as when fragile molds are used, if the tonnage exerted by the press exceeds the structural stability of the mold, the mold may be damaged.
It has proven to be extremely useful in the operation of injection molding machines to measure the final preload force or tonnage applied to the mold by the press. This final tonnage may vary as the press operates due to the heating of the components of the press and due to other factors. The tonnage measurement may be used to adjust the operation of the machine so that the tonnage applied will exceed the force created by the plastic in the mold, and thereby resist mold opening or flashing, but so that the tonnage will not be excessive in order to prevent mold damage or other damage to the apparatus. In addition, the tonnage measurement may be used to adjust the relief valve in the hydraulic supply system. When the hydraulic pressure in the cylinder reaches a maximum value as the mold is fully preloaded, the relief valve could be adjusted downwardly to a lower value to prevent the pressure of the hydraulic fluid in the cylinder and consequently the clamping force from becoming excessive.
In the past, this tonnage has sometimes been measured by mounting a strain gage or transducer on one of the tie bars. The positioning and use of such a transducer have proven to be extremely awkward and unreliable. The placement of the transducer on the tie bar subjects it to being dislodged when the mold is inserted into or removed from the press. In addition, the strain gage is subject to bending action of the tie bar which results in inaccurate readings, since the strain gage does not merely read the tension in the tie bar, but also the bending action.
Apparatus to control the operation of a toggle press is disclosed in U.S. Pat. Nos. 3,642,405 and 3,891,774, both issued to Eggenberger et al. In the apparatus disclosed in these patents, the effective length of the tie bar is changed in order to control the force required to close the press. This force is measured by a dynamometer inserted in the hydraulic feed line to the cylinder. Tonnage is not measured directly in this apparatus, however, and therefore the advantages of measuring the tonnage and providing this measurement to the operator of the machine are not provided. In addition, since the Eggenberger et al. apparatus uses a dynamometer in the hydraulic control line, it assumes a constant clamp closing speed, and therefore is not easily adapted to measure tonnage when the clamp closing speed varies, and thus the speed of the hydraulic fluid in the supply line to the cylinder also varies.